The present invention relates to energy focusing surfaces, such as radio wave antennas, solar concentrators, and the like and is particularly directed to a screw motion-driven tensegrity support architecture, that is configured to stably deploy and adjustably control characteristics of the energy-focusing surface.
The field of deployable structures, such as space-deployed platforms, has matured significantly in the past decade. What once was a difficult art to master has been developed into a number of practical applications by commercial enterprises. The significance of this maturity has been the reliable deployment of various spacecraft-supported antenna systems, similar to that employed by the NASA tracking data and relay satellite (TDRS). In recent years, the development of parabolic, mesh-surface, reflector geometries has been accompanied by improvements in phased arrays (flat panel structures with electronically steered beams), both of which are critical to commercial and defense space programs. As commercial spacecraft production has exceeded military/civil applications, there is currently a demand for structural systems with proven reliability and performance, and the ever present reduced cost. As described in the text by Larson and Wertz, entitled xe2x80x9cSpace Mission Analysis and Design,xe2x80x9d Microcosm, Inc., ISBM: 1881883019; November 1992; 2nd Edition, a spacecraft system""s requirements may be defined through a process of identifying broad objectives, reasonably achievable goals, and cost constraints. Although space missions vary greatly, and the requirements, goals, and costs associated with each task also vary greatly, one constraint is always present: xe2x80x9cspace is expensive.xe2x80x9d
The mission objective for a large, deployable space antenna is to provide reliable radio frequency (RF) energy reflection to an electronic collector (feed) located at the focus of the parabolic surface. The current state of deployable parabolic space antenna design is principally based on what may be termed a segmented construction approach which, as shown in FIGS. 1-4, is configured much like an umbrella. In this type of design, a plurality of radial ribs or segments 1 are connected to a central hub 3, that supports an antenna feed 5. A mechanical advantaged linear actuator (not shown) is used to drive the segments 1 from their stowed or unfurled condition, shown in the diagrammatic side and end views of FIGS. 1 and 2, into a locked, over-driven, position, so as to deploy a surface 7, as shown in the diagrammatic side and end views of FIGS. 3 and 4. A shortcoming of a single fold design of this type of antenna is the fact that the height of the stowed package is over one half of the deployed diameter. Other proposals include the use of hoop tensioners and mechanical memory surface materials.
To meet the above-stated objective, an analysis of mathematics and electrical engineering yields three fundamental parameters of the antenna: 1-defocus, 2-mispointing, and 3-surface roughness. As diagrammatically illustrated in FIG. 5, for a receiving antenna, defocus is defined as the error the surface of a reflector 10 that causes the received energy 12 to paint or be projected upon an area 14, rather than converge onto a focal point (where an antenna feed is placed). As shown in FIG. 6, mispointing corresponds to the misplacement of the converged energy 12 to a spatial position 16 other than the designed focal point 18. The third characteristicxe2x80x94surface roughness (or the approximation of a prescribed (e.g., parabolic) surface geometry), defines the reflector""s ability to reflect and collect a given band of RF energy. Higher band reflectors require a more accurate surface that better approximates the theoretical parabola. Conversely, for a transmitting antenna, defocus produces divergent (rather than parallel) waves of energy from the reflector surface, while mispointing directs these waves in the wrong direction.
In recent years, numerous Defense Department organizations have solicited for new approaches to deployable antenna structures. The Air Force Research Laboratories (AFRL) are interested in solutions to aid with their Space Based Laser and Radar programs, and have requested new solutions to building precision deployable structures to support the optical and radar payloads. These requests are based upon the premise that the stowed density for deployable antennas can be significantly increased, while maintaining the reliability that the space community has enjoyed in the past. A failure of these structures is unacceptable. If the stowed volume can be reduced (therefore an increase in density for a given weight), launch services can be applied more efficiently.
The implementation of multiple vehicle launch platforms (e.g., the Iridium satellite built by Motorola) has presented a new case where the launch efficiency is a function of the stowed spacecraft package, and not the weight of the electronic bus. For extremely high frequency (EHF) systems (greater than 20 GHz) in low earth orbit (LEO), the antenna aperture needs to be only a few meters in diameter. However, for an L-band, geosynchronous orbit satellite (such AceS built by Lockheed Martin) the antenna aperture diameter is fifty feet. Less weight and payload drag must be achieved to ensure a more efficient assent into a geosynchronous orbit.
A relatively comprehensive study of the technology needs for future space systems to be published in the last decade was released by the International Technology Research Institute in a WTEC Panel Report entitled: xe2x80x9cGlobal Satellite Communications Technology and Systems, Executive Summary,xe2x80x9d Nov. 11, 1998. This NSF/NASA sponsored research commissioned a panel of U.S. satellite engineers and scientists to study international satellite RandD projects to evaluate the long-term presence of the United States in this industry. A prior study was undertaken in 1992 to establish that there was significant activity in Europe and Asia that rivaled that of the U.S., and benchmarked this RandD to U.S. capability. The later study added market, regulatory, and policy issues in addition to the technology developments. The conclusion was that while the U.S. holds a commanding lead in the space marketplace, there are ongoing gains by both continents. This is evident in space launch, where Ariane Space has nearly achieved the capabilities of Boeing""s (Delta) rocket services.
Once significant aspect of this study is that U.S. manufacturers are meeting their goals for short-term research (achieving program performance), but have greatly neglected the long-term goals, which has traditionally been funded by the government. A top candidate technologies include structural elements, materials and structures for electronic devices, and large deployable antennas (having diameters in excess of twenty-five meters). While there have been fourteen meter subsystems developed to meet geosynchronous system requirements during the 1990s, the large deployable requirement has yet to be addressed or developed.
Tetrobots have been developed in the last few years as a new approach to modular design. The tetrobot approach, which is described in the text by G. Hamlin et al, entitled: xe2x80x9cTETROBOT, A Modular Approach to Reconfigurable Parallel Robotics,xe2x80x9d Kluwer Academic Publishers, 1998 (ISBN: 0-7923-8025-8) utilizes a system of hardware components, algorithms, and software to build various robotic structures to meet multiple design needs. These structures are Platonic Solids (tetrahedral and octahedral modules), with all the connections made with truss members. As described in the text by P. Tidwell et al, entitled: xe2x80x9cKinematic Analysis of Generalized Adaptive Trusses,xe2x80x9d First Joint U.S./Japan Conference on Adaptive Structures, Nov. 13-15, 1990, Technomic Publishing Co., pp. 772-791, adaptive trusses have been applied to the field of deployable structures, providing the greatest stiffness and strength for a given weight of any articulated structure or mechanism. Using the tetrahedron geometry (6-struts and 4-vertices) as its basis, the Tidwell et al text proposes a series of octahedral cells (12-struts and 6-vertices) to build an adaptive structure. An article by B. Wada et al, entitled: xe2x80x9cUsing Adaptive Structures to Enable Future Missions by Relaxing Ground Test Requirements, Journal of Spacecraft, Vol. 28, No. 6, November-December 1991, pp. 663-669 concludes that from well defined forward analyses (position, velocity and acceleration), this adaptive truss would be useful for deployed structures to remove position or motion errors caused by manufacturing, temperature change, stress, or external force.
The most complex issue in developing a reliable deployable structure design is the packaging of a light weight subsystem in as small a volume as possible, while ensuring that the deployed structure meets system requirements and mission performance. An article by D. Warnaar, entitled: Evaluation Criteria for Conceptual of Deployable-Foldable Truss Structures,xe2x80x9d ASME Design Engineering: Mechanical Design and Synthesis, Vol. 46, pp. 167-173, 1992, in describing criteria developed for deployable-foldable truss structures, addresses the issues of conceptual design, storage space, structural mass, structural integrity, and deployment. This article simplifies the concepts related to a stowed two-dimensional area deploying to a three-dimensional volume. A tutorial on deployable-foldable truss structures is presented in: xe2x80x9cConceptual Design of Deployable-Foldable Truss Structures Using Graph Theory-Part 1: Graph Generation,xe2x80x9d by D. Warnaar et al, ASME 1990 Mechanisms Conference, pp. 107-113, September 1990, and xe2x80x9cConceptual Design of Deployable-Foldable Truss Structures Using Graph Theory-Part 2: Generation of Deployable Truss Module Design Concepts, by D. Warnaar et al, ASME, 1990 Mechanisms Conference, pp. 115-125, September 1990. This series of algorithms presents a mathematical representation for the folded (three-dimensional volume in a two-dimensional area) truss, and aids in determining the various combinations for a folded truss design.
NASA""s Langley Research Center has extensive experience in developing truss structures for space. One application, a 14-meter diameter, three-ring optical truss, was designed for space observation missions. An article by K. Wu et al, entitled: xe2x80x9cMulticriterion Preliminary Design of a Tetrahedral Truss Platform,xe2x80x9d Journal of Spacecraft and Rockets, Vol. 33, No. 3, May-June 1996, pp. 410-415, details a design study that was performed using the Taguchi methods to define key parameters for a Pareto-optimal design: maximum structural frequency, minimum mass, and the maximum frequency to mass ratio. In the study, tetrahedral cells were used for the structure between two precision surfaces. 31 analyses were performed on 19,683 possible designs with an average frequency-to-mass ratio between 0.11 and 0.13 Hz/kg. This results in an impressive 22 to 26 Hz for a 200-kg structure.
The field of deployable space structures has proven to be both technically challenging and financially lucrative during the last few decades. Such applications as large parabolic antennas require extensive experience and tooling to develop, but is a key component to the growing personal communications market. Patents relating to deployable space structures have typically focused on the deployment of general truss network designs, rather than specific antenna designs. Some of these patents address new approaches that have not been seen in publication.
For example, the Kaplan et al, U.S. Pat. No. 4,030,102, and Waters et al, U.S. Pat. No. 4,825,225 describe the application of strut and tie construction to deployable antennas. However, the majority of patents address trusses and the issues associated with their deployment and minimal stowage volume. For example, the Nelson U.S. Pat. No. 4,539,786 describes a design for a three-dimensional rectangular volume based on an octahedron. Deployment uses a series of ties within the truss network, and details of the joints and hinges are described. When networked with other octahedral subsets, a compact stow package could be expanded into a rigid three-dimensional framework.
Other patents described continued work in expandable networks to meet the needs of International Space Station. For example, the Natori U.S. Pat. No,. 4,655,022, employs beams and triangular plates to form tetrahedral units that provide a linear truss. The patent details both joint and hinge details and the stowage and deployment kinematics. Similarly, the Kitamura et al, U.S. Pat. No. 5,085,018, describes a design based on triangular plates, hinged cross members, and ties to build expanding masts from very small packages.
A series of Onoda U.S. Pat. Nos. 4,667,451, 4,745,725, 4,771,585, 4,819,399 and 5,040,349 and an article by Onoda et al, entitled: xe2x80x9cTwo-Dimensional Deployable Hexapod Truss,xe2x80x9d Journal of Spacecraft and Rockets, Vol. 33, No. 3, May-June 1996, pp. 416-421, detail a number of examples of collapsible/deployable square truss units using struts and ties. Some suggested applications included box section, curved frames for building solar reflectors or antennas. The M. Rhodes et al, U.S. Pat. No. 5,016,418, describes a more practical design that uses no ties, but employs hinges to build a rectangular box from a tube stowage volume. In addition, the Krishnapillai U.S. Pat No. 5,167,100 and Skelton U.S. Pat. No. 5,642,590, describe the use of radial struts and strut/tie combinations, respectively.
In accordance with the present invention, advantage is taken of a highly stable structure, known as a tensegrity structure, to compactly stow, deploy and support an energy focusing surface, such as radio wave antenna, solar concentrator, and the like. In addition, through the use of a screw-motion based deployment and positioning drive of the struts of the tensegrity structure, the pitch and thereby the shape of the supported energy-focusing surface may be controllably adjusted, to facilitate compensating corrections for antenna defocus, mispointing, and surface roughness.
Pursuant to a non-limiting but preferred embodiment of the invention, a 6-6 platform structure for deploying and supporting an energy directing surface, such as a parabolic RF electromagnetic antenna, is configured such that the lowest energy state for the platform structure is in a screw-rotated 6-6 tensegrity position. The 6-6 parallel platform structure comprises an upper hexagonal platform and a lower hexagonal base, the perimeter geometry of each of which is defined by a plurality of interconnected tensioned ties. Vertices of the hexagonal tie base and the hexagonal upper tie platform are interconnected by a set of twelve legs, with a pair of legs extending from each vertex of a respective hexagonal platform/base tie set to adjacent vertices of the opposite hexagonal base/platform tie set. Each leg pair includes a compression member or strut and a tension member or tie. Namely, the twelve legs include six compression struts and an alternating set of six tensioned ties, so that opposite ends of each compression strut connect only to tensioned ties.
The stability of this 6-6 structure requires that the sum of the tie tension forces matches the sum of the compression forces in the struts. This structure provides six degrees of freedom, with the struts being controllably adjustable to establish parameters of an antenna surface subsystem that is supported by the 6-6 parallel platform. Since the geometry of each of the base and the platform is a hexagon, the radius from the center of the 6-6 parallel platform structure to the platform coordinates is equal to the length of the platform/base side. Using the base and platform geometries, Plucker line coordinates are calculated to define the length of the legs.
In order to control deployment from a stowed condition and the geometric parameters of the deployed structure, a screw motion-based based drive system is coupled to the struts to realize a relative z/xcex8z motion or pitch p. The pitch, namely the separation between the upper platform and lower base, is defined as the ratio of linear z change to rotation about the z axis, which typically corresponds to the boresight axis of the antenna surface being deployed. A (parabolically) shaped reflective surface, such as a conductive mesh may be supported by an arrangement of cords and ties attached at a plurality of points along its circular perimeter to the six platform vertices of the 6-6 parallel platform structure, so that the operation of the struts will fully deploy the antenna into its intended geometry as the support structure is screw-driven from its stowed configuration to its tensegrity state.
Key parameters associated with successful operation of deployable antennas are defocus, mispointing, and surface roughness. Since the tensegrity structure of the present invention allows control of the component kinematics of the parallel platform structure, in particular the positioning of the six struts, xe2x80x98tuningxe2x80x99 adjustments to the positions of the struts may be made for any or all of these parameters to comply with a prescribed antenna performance specification. Defocus may be addressed by analysis of any xe2x80x98cuppingxe2x80x99 of structural components, given the assumption that there are constant errors inherent in the antenna subsystem once deployed. For a given reflector surface a tolerance is established for an associated performance specification. The initial positions of the struts may be modified as necessary to control the geometry of the antenna reflector perimeter.
Mispointing deals with improper geometry of the surface causing the energy to be directed to the wrong (theoretical) focal point, although the focal point is actually a focal plane, due to energy management issues with RF transmitter/receivers. The ability to control the positions of the struts provides for surface geometry adjustment so as to direct the RF energy within the boundaries of this plane. The use of screw-based differential axial positioning of the platform relative to the base, which enables the antenna reflector surface to direct the RF energy toward respectively different foci. This enables different receiver feeds to be located in the vicinity of a nominal focal point, allowing different beams at different frequencies to be directed to these receivers.
Surface accuracy depends on properties of the reflective surface (e.g., reflective mesh), such as but not limited to non-linear stiffness, and reflective mesh surface material. Positioning and control of the reflector material through vernier adjustment of the strut support structure and/or the tensioning ties may be employed to provide compensation for properties of the surface material. The use of a minimum number of rigid elements (six compression struts) of the 6-6 tensegrity parallel platform structure of the invention also facilitates maximizing stowage density for a fixed spacecraft area or volume.